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I understand how people would be concerned about that, but from an economic perspective it's not likely to shake out the same way.

The reason is that SF is bordered on three sides by water, and on the fourth side by other cities. So in SF there's literally nowhere else for new housing to go but in the already super-expensive city. In economics we call this inelastic supply of available real-estate.

Portland, while it does have its urban growth boundary, doesn't face the same geographical restrictions, so while prices can (and have and will) increase in Portland, it's highly unlikely that it reaches the proportions of SF.

One driver for this effect is new construction. In cities like SF, there's basically no space for new construction, and any new construction is done on parcels of land that are themselves already incredibly expensive. In cities without such geographic restrictions, new construction is relatively cheap and easy, allowing new housing to be built to "take the edge off" of the price increases, so to speak.

This inelastic supply for new development was particularly evident during the '07/'08 crash - places with highly inelastic supply, such as SF or New York City experienced some of the biggest crashes in property values, because the inelastic supply had driven prices far beyond "normal".

Cities with plenty of space for expansion, meanwhile, didn't experience the same degree of crash (on average) in the housing market because the relatively elastic supply hadn't caused prices to be driven up as much.

Edit: Sources: See Housing Supply and Housing Bubbles by Glaeser, Gyurko, and Seiz for some empirical research on this topic.

This is a (somewhat) less-technical presentation that I did in grad school on that paper - it might make it a bit easier to digest.